We report the discovery of a new low-mass X-ray binary near the center of the unassociated Fermi GeV gamma-ray source 4FGL J0540.0-7552. The source shows the persistent presence of an optical accretion disk and exhibits extreme X-ray and optical variability. It also has an X-ray spectrum well-fit by a hard power law with a Gamma = 1.8 and a high ratio of X-ray to gamma-ray flux. Together, these properties are consistent with the classification of the binary as a transitional millisecond pulsar (tMSP) in the sub-luminous disk state. Uniquely among the candidate tMSPs, 4FGL J0540.0-7552 shows consistent optical, X-ray, and gamma-ray evidence for having undergone a state change, becoming substantially brighter in the optical and X-rays and fainter in GeV gamma-rays sometime in mid-2013. In its current sub-luminous disk state, and like one other candidate tMSP in the Galactic field, 4FGL J0540.0-7552 appears to always be in an X-ray "flare mode", indicating that this could be common phenomenology for tMSPs.
@article{arxiv.2106.07657,
title = {Multiwavelength evidence for a new flare-mode transitional millisecond pulsar},
author = {Jay Strader and Samuel J. Swihart and Ryan Urquhart and Laura Chomiuk and Elias Aydi and Arash Bahramian and Adam Kawash and Kirill V. Sokolovsky and Evangelia Tremou and Andrej Udalski},
journal= {arXiv preprint arXiv:2106.07657},
year = {2021}
}